Optical position encoder device

ABSTRACT

An optical device that includes a grating (for example a moving reflective grating) and a light source disposed opposing a predetermined side of the grating. The optical device also includes a first reference grating (e.g., a fixed grating) disposed between the light source and the grating, a detector disposed opposing the predetermined side of the grating and a second reference grating (e.g., a fixed grating) disposed between the detector and the grating. The grating, first reference grating and light source are configured for movement relative to one another. For example, the grating can be moveable while the light source and first reference grating are fixed.

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BACKGROUND OF THE INVENTION

The present invention relates to optical encoders, and in particularencoders using a hologram or other grating to read a moving grating,such as a CD.

Many conventional optical position encoder devices (also referred to asposition sensors) illuminate the moving grating thereof using aincoherent light source which includes numerous independent pointsources of different wavelengths. U.S. Pat. No. 4,879,510, U.S. Pat. No.4,472,629 and U.S. Pat. No. 4,691,101, each of which is herebyincorporated in full by reference for all purposes, are examples of suchoptical position encoder devices.

FIG. 1(a) illustrates the basic principles of a conventional opticalposition encoder device 100. In conventional optical position encoder100, an incoherent light source 101 illuminates a moving binary grating102 (with the direction of movement indicated by the arrows). Moreover,a fixed grating 103 with the same period as moving binary grating 102 isplaced adjacent to the moving binary grating. As the moving binarygrating 102 moves across (e.g., along or parallel to) fixed grating 103,the amount of light falling on detector 104 (depicted by a dashed linein FIG. 1(a)) is dependent on the alignments of gratings 102 and 103.The largest amount of light is detected when gratings 102 and 103 arealigned in phase and the least amount of light is detected when grating102 and 103 are aligned out of phase. The contrast of the modulatedlight is dependent on the adjacency (i.e., separation) of gratings 102and 103. If the two grating are apart by more than a few periods of thegratings, detector 104 can not be used to sense the position of movingbinary grating 102.

In order to increase the distance between gratings 102 and 103 beyond afew periods of the gratings, a conventional optical position encoderdevice configuration that includes a lens is used. Such a configurationis shown in FIG. 1(b). In FIG. 1(b), optical position encoder device 110employs a lens 107 between a moving grating 106 and a fixed grating 108.Lens 107 produces an image of moving grating 106 on fixed grating 108.Also illustrated in FIG. 1(b) are an incoherent light source 105 and adetector 109 of optical position encoder device 110.

FIG. 2 shows an optical position encoder device 200 that is similar tooptical position encoder device 110. However, in optical positionencoder device 200, light source 201 and detector 205 are on the sameside of the moving grating 202. Again, a lens 203 images moving grating202 to fixed grating 204 disposed in front of detector 205.

The resolution of the conventional optical position encoder devicesdepicted in FIGS. 1(b), 1(b) and 2 is rather low. At the same time, theoutput signal from such optical position encoder devices is binary andnot sinusoidal. These drawbacks make such conventional optical devicesunsuitable for use as a high-resolution optical position encoder device.

SUMMARY OF THE INVENTION

Optical devices according to the present invention, such as opticalposition encoder devices, have high resolution and can, for example,produce a sinusoidal output signal. The present invention eliminates theneed for a lens in such an optical device by adding a second fixedgrating. This is also accomplished without the need to rely on aself-imaging aspect of a media.

An embodiment of an optical device according to the present inventionincludes a grating (for example a moving reflective grating) and a lightsource (for example, an incoherent light source such as a light emittingdiode or multi-mode semiconductor laser) disposed opposing apredetermined side of the grating. The optical device also includes afirst reference grating (e.g., a fixed first reference grating) disposedbetween the light source and the grating, a detector (e.g., aphotodetector) disposed opposing the predetermined side of the gratingand a second reference grating (e.g., a second fixed reference grating)disposed between the detector and the grating. The grating, firstreference grating and light source are configured for movement relativeto one another. For example, the grating can be moveable while the lightsource and first reference grating are fixed.

Another embodiment of an optical device according to the presentinvention is an optical position encoder that includes a moving gratingwith a period T_(s), and a photodetector with light sensitivecomponents. This optical position encoder also includes a light sourcedisposed on the photodetector chip, a first fixed grating with a spatialperiod T_(r) disposed on the light source and at least one second fixedgrating (for example, a plurality of sinusoidal second fixed gratings)with a period T disposed on the light sensitive components.

Several unique aspects of the an optical devices according to thepresent invention provide for high resolution and a sinusoidal outputsignal including (1) the optical devices can, for example, contain twofixed (reference) gratings in addition to a moving grating, with one ofthe fixed gratings placed in front of the light source (i.e., betweenthe light source and the moving grating) and the other fixed gratingplaced between a detector and the moving grating (e.g., directly on topof the detector); (2) the moving grating can be, for example, separatedfrom the fixed gratings by a distance that is greater than a few periodsof the moving grating and yet no lens is required between the movinggrating and the detector; and (3) the output signal from the detectorcan be, for example, a sinusoidal signal when the fixed grating placedabove the detector is a sinusoidal grating. Furthermore, the periods ofthe fixed gratings can be related to the period of the moving gratingaccording to a mathematical formula (discussed below) that provides foran interference pattern at the detector to be independent of theseparation of the light source and the first fixed (or reference)grating and for there to be no restriction on the separation between thefirst fixed (or reference) grating and the (moving) grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a simplified depiction of a conventional optical positionencoder device.

FIG. 1(b) is a simplified depiction of conventional optical positionencoder device that includes a lens.

FIG. 2 is a simplified depiction of yet another conventional opticalposition encoder with the arrows indicating directions of light traveland the dashed line indicating a centerline of the optical positionencoder.

FIG. 3 is a simplified depiction of an optical device according to thepresent invention with the arrows nearest label 304 indicating movementdirection for element 304.

FIG. 4 is a simplified exploded perspective view of another opticaldevice according to the present invention

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 depicts an embodiment of an optical device 300 (for example, anoptical position encoder device) according to the present invention. Asillustrated in FIG. 3, optical device 300 includes incoherent lightsource 301, a first fixed grating 303, a moving grating 304, a secondfixed grating 305 and a detector 306. The moving grating is a CD or DVDin one embodiment.

Although a one-dimensional incoherent light source is used in thefollowing analysis, the result can be applied to a two-dimensional lightsource as well. In FIG. 3, point source 302 is one of a plurality ofpoint sources included in incoherent light source 301. Arrowed linesoriginating from point source 302 indicate directions of light travel.The wavefront from light source 301, after passing through first fixed(reference) grating 303, is given by $\begin{matrix}{{{u\left( {x,{Z = Z_{1}}} \right)} = {\exp\left\{ \frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}}\quad \right)}^{2}}{\lambda\quad Z_{1}} \right\}*\left\lbrack {a_{0} + {2a_{1}\cos\left\{ \frac{2\pi\quad x}{T_{r}} \right\}}} \right\rbrack}},} & (1)\end{matrix}$where λ is the wavelength of point source 302. The first term in Eq. (1)represents the wavefront from point source 302 and the second termrepresents the transmission of the first fixed (reference) grating 303with period T_(r). The wave from point source 302 propagates through thefirst fixed (reference) grating 303 at plane Z1 (in other words, firstfixed grating 303 is disposed a distance Z₁ from light source 301) tomoving grating 304 with period T_(s) at plane Z2 (which is a distancebetween 303 and 304) and all the way to the detector plane at Z₃. At thedetector plane Z₃, the conditions with which the periodic signal isindependent of the source location x′ can be determined. The function inEq. (1) can be rewritten as follows: $\begin{matrix}{{{u\left( {x,t,{Z = Z_{1}}} \right)} = {{a_{0}\exp\left\{ \frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}}\quad \right)}^{2}}{\lambda\quad Z_{1}} \right\}} + {a_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad + {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad Z_{1}} + {{\mathbb{i}}\quad 2\quad\pi\quad{x^{\prime}/T_{r}}} - \left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}} \right\}}}}{{u\left( {x,t,{Z = Z_{1}}} \right)} = {{a_{0}\exp\left\{ \frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}}\quad \right)}^{2}}{\lambda\quad Z_{1}} \right\}} + {a_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad + {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad Z_{1}} + {{\mathbb{i}}\quad 2\quad\pi\quad{x^{\prime}/T_{r}}} - \left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}} \right\}} + {a_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad - {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad Z_{1}} - {2{\mathbb{i}}\quad\pi\quad{x^{\prime}/T_{r}}} - \left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}} \right\}}}}} & (2)\end{matrix}$

Eq. (2) indicates that the first fixed grating 303 divides the lightsource into three light sources locating at x=0 and x=x′. The waves fromthese three point light sources then reach moving grating 304 withperiod Ts at a distance Z₂ from first fixed grating 303. Although it isassumed for the sake of discussion that moving grating 304 moves withrespect to the light source 302 and the first fixed (reference) grating303, the result is the same when the light source 302 and first fixedreference grating 303 move with respect to the moving grating 304. Thewaves incident on moving grating 304 are obtained by replacing theparameter Z₁ in the denominators in Eq. (2) by (Z₁+Z₂). The result isshown below: $\begin{matrix}{{u\left( {x,{Z = {Z_{1} + Z_{2}}}} \right)} = {{a_{0}\exp\left\{ \frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}}\quad \right)}^{2}}{\lambda\quad\left( {Z_{1} + Z_{2}} \right)} \right\}} + {a_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad + {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad\left( {Z_{1} + Z_{2}} \right)} + \frac{2\quad\pi\quad x^{\prime}}{T_{r}} - \left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}} \right\}} + {a_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad - {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad\left( {Z_{1} + Z_{2}} \right)} - \frac{2\pi\quad x^{\prime}}{T_{r}} - \left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}} \right\}}}} & (3)\end{matrix}$

The wavefront after passing through the moving grating 304 at Z=Z₁+Z₂ isgiven by $\begin{matrix}{{S\left( {x,t,{Z = {Z_{1} + Z_{2}}}} \right)} = {{u\left( {x,{Z = {Z_{1} + Z_{2}}}} \right)}*\left\lbrack {b_{0} + {b_{1}\cos\left\{ \frac{2{\pi\left( {x - {\beta\quad t}} \right)}}{T_{s}} \right\}}} \right\rbrack}} & (4)\end{matrix}$where t represents time. The second term in Eq. (4) is the transmissionof the moving grating 304 moving at a rate of β/T_(s). The transmittedwaves in Eq. (4) can be separated into six different waves. However, theanalysis continues with only one of the terms shown below and alsoleaving out the constant phase term $\begin{matrix}{{\left( \frac{\lambda\quad Z_{1}}{T_{r}} \right)^{2}\text{:}{S_{1}\left( {x,t,{Z = {Z_{1} + Z_{2}}}} \right)}} = {{a_{1}b_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}\quad + {\lambda\quad{Z_{1}/T_{r}}}} \right)}^{2}}{\lambda\quad\left( {Z_{1} + Z_{2}} \right)} + \frac{{\mathbb{i}}\quad 2\quad\pi\quad x^{\prime}}{T_{r}} - {\frac{{{\mathbb{i}}\quad 2\quad\pi}\quad}{T_{s}}\left( {x - {\beta\quad t}} \right)}} \right\}} = {a_{1}b_{1}\exp\left\{ {{\frac{{\mathbb{i}}\quad\pi}{\lambda\quad\left( {Z_{1} + Z_{2}} \right)}\left\lbrack {\left( {x - x^{\prime}} \right) + \left( {\frac{\lambda\quad Z_{1}}{T_{r}} - \frac{\lambda\quad\left( {Z_{1} + Z_{2}} \right)}{T_{s}}} \right)} \right\rbrack}^{2} + {{\mathbb{i}}\quad 2\quad\pi\quad{x^{\prime}\left( {\frac{1}{T_{r}} - \frac{1}{T_{s}}} \right)}} - \frac{{{\mathbb{i}}\quad 2\quad{\pi\beta}\quad t}\quad}{T_{s}}} \right\}}}} & (5)\end{matrix}$

This wave continues to propagate to the second fixed grating 305 whichis at a distance Z₃ (i.e., at plane Z3) from the moving grating 304. Thewavefront at second fixed grating 305 is obtained by replacing theparameter (Z₁+Z₂) by (Z₁+Z₂+Z₃): $\begin{matrix}{{S_{2}\left( {x,t,{Z = {Z_{1} + Z_{2} + Z_{2}}}} \right)} = {{a_{1}b_{1}\exp\left\{ {{\frac{{\mathbb{i}}\quad\pi}{\lambda\quad\left( {Z_{1} + Z_{2} + Z_{3}} \right)}\left\lbrack {\left( {x - x^{\prime}} \right) + \left( {\frac{\lambda\quad Z_{1}}{T_{r}} - \frac{\lambda\quad\left( {Z_{1} + Z_{2}} \right)}{T_{s}}} \right)} \right\rbrack}^{2} + {{\mathbb{i}}\quad 2\quad\pi\quad{x^{\prime}\left( {\frac{1}{T_{r}} - \frac{1}{T_{s}}} \right)}} - \frac{{{\mathbb{i}}\quad 2\quad{\pi\beta}\quad t}\quad}{T_{s}}} \right\}} = {a_{1}b_{1}\exp\left\{ {\frac{{\mathbb{i}}\quad{\pi\left( {x - x^{\prime}} \right)}^{2}}{\lambda\quad\left( {Z_{1} + Z_{2} + Z_{3}} \right)} + {\frac{{\mathbb{i}}\quad 2{\pi\left( {x - x^{\prime}} \right)}}{\left( {Z_{1} + Z_{2} + Z_{3}} \right)}\left( {\frac{Z_{1}}{T_{r}} - \frac{\quad\left( {Z_{1} + Z_{2}} \right)}{T_{s}}} \right)} + {{\mathbb{i}}\quad 2\quad\pi\quad{x^{\prime}\left( {\frac{1}{T_{r}} - \frac{1}{T_{s}}} \right)}} - \frac{{{\mathbb{i}}\quad 2\quad{\pi\beta}\quad t}\quad}{T_{s}}} \right\}}}} & (6)\end{matrix}$

The interference between the wave in Eq. (6) and the 0th order wave atplane Z₃ is given byI(x,t,Z=Z ₁ +Z ₂ +Z ₃)=|a ₀ b ₀ +S(x,t,Z=Z ₁ +Z ₂ +Z ₃)+S*(x,t,Z=Z ₁ +Z₂ +Z ₃)|²=(a ₀ b ₀)²+2a ₀ b ₀ a ₁ b ₁ cos Φ+(a ₁ b ₁)² cos² Φ  (7)where $\begin{matrix}{\Phi = \left\{ {{2{\pi\left( {{\frac{x}{\left( {Z_{1} + Z_{2} + Z_{3}} \right)}\left( {\frac{Z_{1}}{T_{r}} - \frac{\quad\left( {Z_{1} + Z_{2}} \right)}{T_{s}}} \right)} - \frac{\beta\quad t}{T_{s}}} \right)}} + {2\quad\pi\quad x^{\prime}\left\{ {\frac{1}{\left( {Z_{1} + Z_{2} + Z_{3}} \right)}\left( {\frac{Z_{3}}{T_{s}} - \frac{\quad\left( {Z_{2} + Z_{3}} \right)}{T_{r}}} \right)} \right\rbrack}} \right\}} & (8)\end{matrix}$

Therefore, the interference pattern on the detector (plane) 306 isindependent of x′ when $\begin{matrix}{T_{r} = {\frac{\left( {Z_{2} + Z_{3}} \right)}{Z_{3}}{T_{s}.}}} & (9)\end{matrix}$

When the interference pattern on the detector plane is independent ofx′, the addition of more point sources will increase the signalamplitude and at the same time will not affect the contrast of theinterference pattern. The period T of the interference pattern on thedetector plane 306 under the condition given in Eq. (9) is given by$\begin{matrix}{T = {\frac{\left( {Z_{2} + Z_{3}} \right)}{Z_{2}}T_{s}}} & (10)\end{matrix}$

From Eq. (9) and Eq. (10) it can further be shown that $\begin{matrix}{{\frac{1}{T} + \frac{1}{T_{r}}} = \frac{1}{T_{s}}} & (11)\end{matrix}$

This result is very interesting and beneficial in that:

(1) The interference pattern at the detector plane is independent of Z₁,which is the separation between the light source 301 and the first fixedgrating 303.

(2) For Z₂=Z₃, the grating period T_(r) and the detector grating periodT are equal to each other and are also equal to 2T_(s).

(3) When Z₂=Z₃, there is no restriction on the separation between movinggrating 304 and first fixed grating 303.

(4) Eq. (11) shows that the relationship between the periods of thegratings is independent of their respective distances. Once T_(s), T andT_(r) are known, the distances Z₂ and Z₃ can be determined using Eq. (9)and Eq. (10).

(5) Although a monochromatic light source is used in this analysis,neither the grating periods T, T_(r) and T_(s) nor the distancesseparating the gratings depend on the wavelength of the light source.Therefore, any suitable incoherent light source, such as an extendedincoherent light source, can be used.

In the circumstance that the period T of the grating on top of thedetector (i.e., second fixed grating 305) and the period T_(s) of themoving grating 304 are predetermined and, therefore, known, the value ofT_(r) can be determined from Eq. (11). In addition, if it is given thatZ₃=Z₂+Δ, it can be shown that $\begin{matrix}{Z_{2} = {\frac{\Delta\quad T_{s}}{\left( {T - {2T_{s}}} \right)}.}} & (12)\end{matrix}$

Assuming T=43 μm and T_(s)=18.75 μm and Δ=0.54 mm, it can be found thatT_(r) 33.247 μm and Z₂=1.841 mm.

Because gratings 303 and 304 can, for example, contain numerous harmoniccomponents, it is desirable that the second fixed grating 305 near thedetector plane 306 be able to filter out the desired harmonic signal.

FIG. 4 is a simplified exploded view of another embodiment of an opticaldevice 400 (namely and optical position encoder device) in accordancewith the present invention. Optical device 400 included an extendedlight source 401 that is disposed (i.e., mounted) on a photodetector 405and configured to illuminate a first fixed grating 402. Photodetector405 includes light sensitive components 406, 407, 408, and 409.

Optical device 400 also includes moving grating 403 and a plurality ofsecond fixed gratings (not shown in FIG. 4) that are etched directly ontop of light sensitive elements 406, 407, 408, 409 of photodetector 405.When optical device 400 is in use, light reflecting from moving grating403 is incident on the second fixed gratings.

In the embodiment of FIG. 4, the second fixed gratings on top of thelight sensitive elements 406 and 408 are sinusoidal gratings and have a90 degree phase shift with respect to each other. In addition, secondfixed gratings on top of the light sensitive elements 407 and 409 aresinusoidal gratings and have a 180 degree phase shift with respect tothe sinusoidal gratings on top of the light sensitive elements 406 and408. Because of the sinusoidal shape of the second fixed gratings,output signals from each of the light sensitive elements of thephotodetector 405 beneficially receive only one harmonic component.

As will be understood by those of skill in the art, the presentinvention could be embodied in other specific forms without departingfrom the essence of the invention. For example, a moving grating (e.g.,CD or DVD) could be used that is transparent to the wavelengths of lightfrom the light emitter, with the second reference grating andphotodetector being mounted on the far side of the moving grating,opposite the light emitter. Accordingly, the foregoing description isillustrative, but not limiting, of the scope of the invention which isset forth in the following claims.

1. An optical device comprising a primary grating; a light sourcedisposed opposing a predetermined side of the primary grating; a firstreference grating disposed between the light source and the primarygrating; a photodetector disposed opposing the predetermined side of theprimary grating; and a second reference grating disposed between thephotodetector and the primary grating; wherein the primary grating, thefirst reference grating and the light source are configured for movementrelative to one another.
 2. The optical device of claim 1, wherein theprimary grating is a moving grating and the first reference grating andsecond reference grating are fixed gratings.
 3. The optical device ofclaim 1, wherein the primary grating, light source, first referencegrating, second reference grating and photodetector are configured as anoptical position encoder device.
 4. The optical device of claim 1,wherein the grating is a reflective grating.
 5. The optical device ofclaim 1, wherein the first reference grating and second referencegrating are configured for identical relative motion with respect to theprimary grating.
 6. The optical device of claim 1, wherein the lightsource is a semiconductor laser.
 7. The optical device of claim 1,wherein the light source is an extended light source.
 8. The opticaldevice of claim 7, wherein the extended light source is a light emittingdiode (LED).
 9. The optical device of claim 1, wherein a period Tr ofthe first reference grating and a period T of the second referencegrating are related to a period T_(s) of the primary grating by thefollowing formula:${\frac{1}{T} + \frac{1}{T_{r}}} = {\frac{1}{T_{s}}.}$
 10. An opticalposition encoder device comprising: a moving grating with a periodT_(s); a photodetector with light sensitive components; a light sourcedisposed on the photodetector; a first fixed grating with spatial periodT_(r) disposed on the light source; and at least one second fixedgrating with period T disposed on the light sensitive components;wherein the moving grating is moveable relative to the first fixedgrating and the light source.
 11. The optical position encoder device ofclaim 9, wherein the light source is an incoherent light source.
 12. Theoptical position encoder device of claim 9, wherein${\frac{1}{T} + \frac{1}{T_{r}}} = {\frac{1}{T_{s}}.}$
 13. The opticalposition encoder device of claim 10, wherein there is a plurality ofsecond fixed gratings with a fixed phase relationship thereamong suchthat the photodetector receives only one harmonic component.
 14. Theoptical position encoder device of claim 13, wherein the plurality ofsecond fixed gratings are sinusoidal fixed gratings.
 15. An opticaldevice comprising a primary grating; a light source disposed opposing apredetermined side of the primary grating; a first reference gratingdisposed between the light source and the primary grating; aphotodetector disposed on a far side of the primary grating; and asecond reference grating disposed between the photodetector and theprimary grating; wherein the primary grating, the first referencegrating and the light source are configured for movement relative to oneanother.